7. Project Summary/Abstract To achieve the exquisite precision characteristic of synaptic circuits in the mature nervous system, immature synapses form a crude wiring diagram that must remodel during postnatal development. While it is clear that neural activity drives developmental synaptic remodeling, the underlying mechanisms are not fully understood. This K99/R00 proposal will support my career development as I investigate underlying cellular and molecular mechanisms driving activity-dependent synaptic remodeling in the developing brain. I have firmly established that microglia, the resident CNS immune cells, phagocytose synaptic elements in the postnatal brain in response to changes in neural activity. These data raise the intriguing possibility that microglia are a cellular mechanism driving activity-dependent synaptic remodeling. However, it is unknown whether microglia actively phagocytose intact synapses or passively phagocytose synaptic remnants. In addition, the molecular mechanisms underlying activity-dependent microglia-synapse interactions are unknown. These questions will be addressed during the mentored phase of the award at Boston Children's Hospital and Harvard Medical School under the guidance of Dr. Beth Stevens and Dr. Michael Greenberg. Aim 1 is designed to test the hypothesis that microglia are actively phagocytosing intact synapses in response to changes in neural activity. In doing so, I will learn viral-mediated gene delivery to fluorescently label specific neural circuits and 2-photon in vivo live imaging to analyze microglia-synapse interactions in real time. Aim 2 is designed to investigate molecular mechanisms underlying these interactions. After genetic and proteomic screens, I have identified a candidate, interleukin 12 (IL-12). I will test the hypothesis that IL-12 regulates activity-dependent microglia-synapse interactions by in vivo imaging in IL-12 and IL-12 receptor KO mice. Specific Aim 3 (to be completed in my own laboratory) will determine the functional significance of activity- dependent microglia-synapse interactions by testing the hypothesis that these interactions regulate the functional development of synaptic circuits. I will genetically ablate microglia during a specific window of postnatal development and assess synapse structure and physiology. To manipulate microglia function more specifically, I will also test the role of the IL-12 pathway. Furthermore, as an alternative and/or future direction, my genetic and proteomic screens have identified several other putative molecular pathways that I will test. Given that aberrant synaptic circuits and microglial dysfunction have now been linked with several neurodevelopmental and psychiatric disorders, this proposal will have broad implications. My long term career goal as an independent investigator is to translate my findings to understand how microglia and other glial cell types may contribute to synaptic circuit abnormalities associated with disorders such as autism and schizophrenia. This award will help me to complete my training and provide an opportunity to learn techniques and obtain data for a successful R01 application to fund my future work.